Set focus back on a ComboBox if the value is incorrect

I am trying to set my ComboBoxes so that the user can either choose from the list or set their own value (the combo box is for a custom resolution so there will be default values or they can give their own).

I am trying to make it so that if their value is incorrect (below 0 or not an Int) then it shows a tooltip and prevents it from losing focus. Here is my code: